Computer-implemented methods for detecting and/or sorting defects in a design pattern of a reticle

ABSTRACT

Various computer-implemented methods are provided. One method for sorting defects in a design pattern of a reticle includes searching for defects of interest in inspection data using priority information associated with individual defects in combination with one or more characteristics of a region proximate the individual defects. The priority information corresponds to modulation levels associated with the individual defects. The inspection data is generated by comparing images of the reticle generated for different values of a lithographic variable. The images include at least one reference image and at least one modulated image. A composite reference image can be generated from two or more reference images. The method also includes assigning one or more identifiers to the defects of interest. The identifier(s) may include, for example, a defect classification and/or an indicator identifying if the defects of interest are to be used for further processing.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/005,658 entitled “Computer-Implemented Methods for Detecting and/orSorting Defects in a Design Pattern of a Reticle,” filed Dec. 7, 2004,now U.S. Pat. No. 7,729,529 issued Jun. 1, 2010, both of which areincorporated by reference as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to computer-implemented methodsfor detecting and/or sorting defects in a design pattern of a reticle.Certain embodiments relate to a computer-implemented method thatincludes generating a composite reference image from two or morereference images and using the composite reference image for comparisonwith other sample images for defect detection. Other embodiments includesorting defects using priorities, defect attributes, defect appearanceand background information. Additional embodiments relate to assistingthe user in locating the relevant and unique defects based on backgroundappearance and other characteristics combined with wafer design data andknowledge of process modulation.

2. Description of the Related Art

The following description and examples are not admitted to be prior artby virtue of their inclusion in this section.

The rapid decrease in k₁ (line-width=k₁ (λ/NA)) in lithographicmanufacture of semiconductor devices has necessitated the use ofResolution Enhancement Techniques (RET). These RET include, but are notlimited to, Optical Proximity Corrections (OPC), Phase Shift Masks(PSM), and assist bar corrections. Although they are implemented insemiconductor device designs to facilitate low k₁ lithography, these RETmake reticles more difficult and consequently more expensive tomanufacture.

Semiconductor device design and reticle manufacturing quality areverified by different procedures before the reticle enters asemiconductor fabrication facility to begin production of integratedcircuits. The semiconductor device design is checked by softwaresimulation to verify that all features print correctly after lithographyin manufacturing. Such checking is commonly referred to as “Design RuleChecking.” The output of this design rule checking can produce apotentially large set of critical points, sometimes referred to as “hotspots” on the reticle layout. This set can be used to direct apoint-to-point inspector, such as at a Review SEM, but this can behighly inefficient due to the number of critical points. The reticle isinspected at the mask shop for reticle defects and measured to ensurethat the features are within specification. Marginal RET designs notnoted by simulation checks translate into electrical failures in waferfabrication, affect yield, and possibly remain unnoticed until waferfabrication is complete.

Traditional methods employed in the inspection of complex mask patternsplace tremendous demand on reticle inspection tools. One technique forperforming image qualification entails using focus exposure matrixtechniques. Performing an inspection of a conventional focus exposurematrix introduces a complication in that every exposure field isdifferent. Die-to-die comparison is performed between adjacent localexposure fields. Any pattern change that may occur at a defocus positionthat is physically located farther than one exposure field from thenominal exposure field will not, therefore, be detected as differentbecause the nominal exposure field is no longer factored in thecomparison. Moreover, current reticle inspection techniques cannotdetect the presence of an error in the design database. Prior art singledie reticle inspection entails implementation of a design simulationtechnique in which a signal derived from an actual reticle is subtractedfrom a simulated design reference.

What is needed, therefore, is an inspection technique that is effectivein locating pattern anomalies in a single die or a multi-die reticle anddetecting reticle design errors resulting from errors in the design database.

Methods have been invented to address the above-described needs. Thesemethods are often referred to as “Process Window Qualification” Methodsor “PWQ” Methods and are described in U.S. Patent ApplicationPublication No. US2004/0091142 to Peterson et al., which is incorporatedby reference as if fully set forth herein. Software packages that areconfigured to perform methods such as those described by Peterson et al.are commercially available from KLA-Tencor, San Jose, Calif. In general,the methods can be used to find design elements of a reticle that willfail in lithographic processing when used with lithographic variables(e.g., focus, dose, etc.) that are within a normal process window forthe reticle.

PWQ methods are often performed using wafer inspection tools such as anyof the wafer inspection tools that are commercially available fromKLA-Tencor. In one example, a wafer is printed with columns of dies,each containing the design pattern on the reticle, in an N-M-N pattern.The “N” dies are those dies that are printed with a “nominal”lithographic variable (which may also be commonly referred to as a“nominal lithography parameter,” a “nominal lithographic processparameter,” or a “nominal process condition”). The “M” dies are printedwith a value of the lithographic variable that is different than thenominal lithographic variable. In other words, the M dies are printedwith a modulated lithographic variable. The nominal lithographicparameter may be the value of the lithographic parameter known torepresent the “best condition” for exposure of a wafer with the reticle.Alternatively, the nominal lithographic parameter may be assigned adifferent baseline value of the lithographic parameter. The lithographicvariable can be modulated positively and negatively with respect to thenominal lithographic variable in rows of dies printed on the wafer.

After exposure of the wafer with the reticle, the wafer is inspected bycomparing the modulated die to the two nominal dies on either side ofthe modulated die. Adjacent dies are compared after both of the adjacentdies have been imaged. Therefore, the comparison is performedsequentially in the order in which the dies are imaged. Differencesbetween the adjacent dies can be stored as potential defects.

Positively modulated dies and negatively modulated dies may be handledseparately for purposes of analysis. In addition, the defects that aredetected in the modulated dies may be analyzed to determine the priorityor relevance of the defects. Furthermore, the user may be able to reviewthe defects to find the critical or important defects that weredetected.

Although the above-described PWQ methods have proved successful inmeeting the needs outlined above, these methods can also be improved.For example, in the inspection process, the modulated dies are comparedto exactly two nominal or reference dies. Randomly occurring defects ineither or both of the reference dies may adversely affect the results ifthey result in reducing the priority of defects in the modulated dies.In addition, using a three die comparison (i.e., two reference dies foreach modulated die) results in the use of most of the wafer area forprinting the reference dies.

In the PWQ software used today, potential failure points in the designpattern are identified by looking for repeating defects. Unfortunately,by its very nature, the experiment can produce an overwhelming number ofunimportant repeating defects, particularly in the dies that are highlymodulated. Automatic defect classification (ADC) is one way to reducethe number of candidate defects. However, the inline ADC (iADC) methodthat is available for PWQ uses additional information about the defectitself, and much of this information is irrelevant to finding the mostlikely failure points. A newer version of the iADC method as describedin U.S. patent application Ser. No. 10/954,968 to Huet et al., now U.S.Pat. No. 7,142,992 issued Nov. 28, 2006, which is incorporated byreference as if fully set forth herein, provides the capability offocusing on background features. However, in these methods, a userselects background features from the complete set of available featuresthat are used to classify defects thereby creating an extra step in thesetup of the inspection. Additionally, in current methods for reviewingdefects, it is difficult to obtain multiple examples of potentiallyinteresting defects.

The PWQ methods may also be altered to use a stored “golden die” imagefor comparison to the modulated images. A “golden die” image may begenerally defined as an image of design pattern information on a reticlethat is known in some manner to be free of defects. Therefore, by usinga golden die image, the number of nominal reference dies printed on thewafer may be reduced, or even eliminated, thereby allowing moremodulated dies to be printed on the wafer. However, there aredisadvantages to using such a golden die image. For example, a detailedgolden die image can require hundreds of Gbytes of storage. On the otherhand, the detail of the golden die image may be reduced, butcompromising on the detail of the golden die image compromises theeffectiveness of the inspection method. Furthermore, a golden die imagemost likely is not formed under the same processing conditions as thetest die, particularly if the golden die image is generated bysimulation or if the golden die image was obtained from a differentwafer than the wafer on which the modulated dies are printed. Thedifferences in formation of the golden die and the modulated dies mayresult in false defect detection during inspection of the modulateddies. Moreover, reading the golden image from storage media can beslower that reacquiring the golden image from an image computer oranother computer system.

Accordingly, it may be advantageous to develop computer-implementedmethods for detecting and/or sorting defects in a design pattern of areticle that allows accurate defect detection while using relatively fewnominal reference dies, increases the accuracy of the defect detectionby reducing the adverse effects of defects in the nominal reference dieson the accuracy of the defect detection, allows rapid identification andremoval of unimportant repeating defects so that these defects do notobscure the defects of interest, allows multiple examples of interestingdefects to be found relatively easily, allows classification of defectsin a substantially automated manner, or achieves one or more of theabove improvements without using a stored golden die image of the designpattern on the reticle.

SUMMARY OF THE INVENTION

One embodiment relates to a computer-implemented method for sortingdefects in a design pattern of a reticle. The method includes searchingfor defects of interest in inspection data using priority informationand defect attributes associated with individual defects in combinationwith one or more characteristics of a region proximate the individualdefects and one or more characteristics of defects. The inspection datais generated by comparing images of the reticle generated for differentvalues of a lithographic variable. The images include at least onereference image and at least one modulated image. The method alsoincludes assigning one or more identifiers to the defects of interest.

In one embodiment, the priority information is derived from therelationship between inspected defects and their correspondingmodulation levels. In another embodiment, defect attributes containsimple defect information such as location, size, intensity magnitudeand polarity as well as inspection parameters. Defects are filtered bydefect priorities and attributes. The filtering criteria can be selectedby user. In some embodiments, the one or more characteristics of regionsproximate the defects and on the defects are computed from reference anddefect images, respectively.

In another embodiment, the method may include grouping the defects ofinterest based on the one or more characteristics of the regionproximate the individual defects or the one or more characteristics ofthe defects, or a combination thereof. The characteristics used ingrouping are selected by the user. In a different embodiment, the methodmay include retrieving defects which are similar to given defects basedon defect appearance, attributes and one or more characteristics ofregion proximate the defects. The retrieving criteria can be selected bythe user.

In one embodiment, the one or more identifiers may include a defectclassification. In another embodiment, the one or more identifiers mayinclude an indicator identifying if the defects of interest are to beused for further processing. In one such embodiment, assigning the oneor more identifiers is performed automatically based on the priorityinformation and defect classification.

In an additional embodiment, the method may include comparing thepotential defects of interest to the results generated by design rulechecking performed on design pattern data of the reticle to determine ifthe defects of interest correlate to design rule checking criticalpoints. In one such embodiment, the method may also include removingfrom the inspection data the defects that do not correlate with thecritical points. In a similar manner, the method may include comparingthe potential defects of interest to the results generated by opticalrule checking (ORC) performed on design pattern data of the reticle. Ingeneral, steps described herein involving the use of DRC results mayalternatively be performed using ORC results. Each of the embodiments ofthe method described above may include any other step(s) describedherein.

Another embodiment of the invention relates to a computer-implementedmethod for detecting defects in a design pattern of a reticle. Themethod includes acquiring images of the reticle for different values ofa lithographic variable. The images include two or more reference imagesobtained at nominal values and one or more modulated images. The methodalso includes generating a composite reference image from the two ormore reference images. In addition, the method includes comparing atleast two of the images. The at least two of the images include thecomposite reference image. In one embodiment, the user, with knowledgeof the wafer layout or dies printed on the wafer, informs the systemwhich images will be used for reference (e.g., composite ornon-composite) and for comparison. In this manner, the user may selectthe images that are used for comparison. The method further includesdetermining if a defect is present in the design pattern of the reticleusing results of the comparison.

In some embodiments, the one or more characteristics of the region maybe selected by a user. In another embodiment, simulated images, as fromGDS or simulated aerial images, are used to determine thecharacteristic(s) of the background, based on the location of the defectin the reticle. The characteristic(s) of the region may be extractedfrom such images using any technique known in the art. In addition,experimentally generated aerial images may be used in a similar manner.In a different embodiment, high resolution images of the reticle may beused to determine characteristic(s) of the background region proximatethe defect, based on the location of the defect in the reticle. A highresolution image of the reticle may be obtained using any appropriatehigh resolution imaging system known in the art. For example, severalcommercially available reticle inspection systems are configured to formhigh resolution images of the reticle.

In addition or alternatively, the critical points may be regrouped orfiltered using the “Defects Like Me” function described herein to reducethe population. In this manner, inspecting, measuring, and/or reviewingcritical points that are similar may be identified or eliminated.

In addition, the critical points identified by the DRC may be overlaidwith the inspection data generated as described herein. The inspectiondata may be data generated by imaging a wafer on which one or moremodulated dies and one or more reference dies are printed.Alternatively, the inspection data may include aerial images of thereticle design pattern generated by simulation or experimentation. Inthis manner, the defects of interest found as described herein may becompared to inspection data generated by design rule checking todetermine if the defects of interest correlate to design rule checkingdefects. The design rule checking defects that do not correlate with thedefects of interest may then be removed from the design rule checkinginspection data. In a similar manner, the defects of interest may becompared to data generated by optical rule checking to determine if thedefects of interest correlate to optical rule checking defects.

In a further embodiment, the images may include images of an entireswath of dies printed on a wafer using the reticle. In this embodiment,the at least two images used for the comparison may include images ofall of the dies in the entire swath. In another such embodiment,modulated dies in the entire swath are printed using the same value ofthe lithographic variable, which is different than the value of thelithographic variable at which reference dies are printed in the entireswath. In yet another such embodiment, modulated dies in the entireswath are printed using the different values of the lithographicvariable. In this embodiment, reference dies in the entire swath areprinted using an additional different value of the lithographicvariable.

In some embodiments, acquiring the images includes acquiring images ofthe design pattern printed on a wafer using the reticle. In otherembodiments, the images may include aerial images. Each of theembodiments of the method described above may include any other step(s)described herein.

Another embodiment relates to a different computer-implemented methodfor detecting and sorting defects in a design pattern of a reticle. Thismethod includes acquiring images of the reticle for different values ofa lithographic variable. The method also includes comparing at least twoof the images. In addition, the method includes determining ifindividual pixels are different in the design pattern using results ofthe comparison. The method also includes determining if pixeldifferences in the at least two images follow a typical or atypicaltrend over the different values of the lithographic variable.

If pixel differences are determined to be present, the method mayinclude assigning the location to a group based on comparison to a trendin a plot of one or more characteristics of the images of the defect asa function of the different values of the lithographic variable. Forexample, an atypical trend may be identified as a potentially relevantdefect location. The images used in the method include, in someembodiments, images of modulated dies printed at the different values ofthe lithographic variable and images of reference dies printed using anadditional different value of the lithographic variable.

An additional embodiment relates to another computer-implemented fordetecting defects in a design pattern of a reticle. This method includesacquiring images of an entire swath of dies printed on a wafer using thereticle. At least two of the dies are printed at different values of alithographic variable. The method also includes, subsequent to theacquisition of the images of the entire swath, comparing at least two ofthe images. In addition, the method includes determining if a defect ispresent in the design pattern using results of the comparison.

In one embodiment, the dies include modulated dies and at least onereference die. In another embodiment, the dies may include two or morereference dies as defined in the inspection recipe. In this embodiment,the method may also include generating a composite reference image fromthe images of the two or more reference dies. In such an embodiment, oneof the at least two of the images used for the comparison includes thecomposite reference image. In an additional embodiment, the dies mayinclude modulated dies and at least one reference die. Each of theembodiments of the method described above may include any other step(s)described herein.

Further embodiments relate to a carrier medium that includes programinstructions executable on a computer system to perform any of thecomputer-implemented methods described herein. Additional embodimentsrelate to a system configured to perform any of the computer-implementedmethods described herein. The system may include a processor configuredto execute program instructions for performing one or more of thecomputer-implemented methods described herein. In one embodiment, thesystem may be a stand-alone system. In another embodiment, the systemmay be a part of or coupled to an inspection system such as a waferimaging system or an aerial imaging measurement system. In a differentembodiment, the system may be a part of or coupled to a defect reviewsystem. In yet another embodiment, the system may be coupled to a fabdatabase. The system may be coupled to an inspection system, a reviewsystem, and/or a fab database by a transmission medium such as a wire, acable, a wireless transmission path, and/or a network. The transmissionmedium may include “wired” and “wireless” portions.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages of the present invention may become apparent to thoseskilled in the art with the benefit of the following detaileddescription of the preferred embodiments and upon reference to theaccompanying drawings in which:

FIGS. 1-4 a are schematic diagrams illustrating plan views of differentconfigurations of dies printed on a wafer with a reticle for differentvalues of a lithographic variable;

FIG. 5 is a graph illustrating examples of different trends in plots ofa characteristic of images of defects as a function of different valuesof a lithographic variable;

FIGS. 6-7 are screenshots illustrating examples of different userinterfaces that can be used to sort defects detected by the methodsdescribed herein;

FIG. 8 is a schematic diagram illustrating a side view of one embodimentof a system that can be used to perform one or more of thecomputer-implemented methods described herein; and

FIG. 9 is schematic diagram illustrating a side view of an apparatusthat can be used to acquire aerial images of a design pattern of areticle.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and may herein be described in detail. Thedrawings may not be to scale. It should be understood, however, that thedrawings and detailed description thereto are not intended to limit theinvention to the particular form disclosed, but on the contrary, theintention is to cover all modifications, equivalents and alternativesfalling within the spirit and scope of the present invention as definedby the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As used herein, the term “defect” refers to a defect in a design patternof a reticle that may cause a defect in a design pattern printed on awafer using the reticle such as excessive corner rounding,unsatisfactory dimensions, missing features, bridging between features,etc. In particular, the methods described herein are particularlysuitable for detecting defects in resolution enhancing technology (RET)features of the design pattern.

The terms “reticle” and “mask” are used interchangeably herein. Areticle generally includes a transparent substrate such as glass,borosilicate glass, and fused silica having a layer of opaque materialformed thereon. The opaque regions may be replaced by regions etchedinto the transparent substrate.

Many different types of reticles are known in the art, and the termreticle as used herein is intended to encompass all types of reticles.For example, the term reticle refers to different types of reticlesincluding, but not limited to, a clear-field reticle, a dark-fieldreticle, a binary reticle, a phase-shift mask (PSM), an alternating PSM,an attenuated or halftone PSM, and a ternary attenuated PSM. Aclear-field reticle has field or background areas that are transparent,and a dark-field reticle has field or background areas that are opaque.A binary reticle is a reticle having patterned areas that are eithertransparent or opaque. Binary reticles are different from phase-shiftmasks (PSM), one type of which may include films that only partiallytransmit light, and these reticles may be commonly referred to ashalftone or embedded phase-shift reticles. If a phase-shifting materialis placed on alternating clear spaces of a reticle, the reticle isreferred to as an alternating PSM, an ALT PSM, or a Levenson PSM. Onetype of phase-shifting material that is applied to arbitrary layoutpatterns is referred to as an attenuated or halftone PSM, which may befabricated by replacing the opaque material with a partiallytransmissive or “halftone” film. A ternary attenuated PSM is anattenuated PSM that includes completely opaque features as well.

A reticle, as described herein, may or may not include a pellicle, whichis an optically transparent membrane that seals off the reticle surfacefrom airborne particulates and other forms of contamination. The termreticle may also be used to refer to a reticle that includes opticalproximity correction (OPC) features. OPC features are designed to reducedistortions of an image printed using the reticle by reducing opticalproximity effects. The term “optical proximity effects” generally refersto variations in lateral dimensions or shapes of printed features due tothe proximity of other features on the reticle. Such effects may bereduced by determining the distortions due to the optical proximityeffects and altering the features on the reticle to compensate for suchdistortions.

RET such as OPC are increasingly being applied to integrated circuit(IC) designs in order to print features on device wafers which aresmaller than the wavelength of light used as the exposure source. TheseRETs often involve the addition of extra features to the designincluding sub-resolution assist features (SRAF) and serifs with theresult that the layout of the design on the photomask or reticle becomesextremely complex. Verifying that the RET features will print correctlyon the reticle and that the SRAFs will not print on the wafer but willcause the main features to print correctly on the wafer is becoming anincreasingly difficult task. Furthermore, optical effects such as maskerror enhancement factor (MEEF) may cause additional distortion of thefinal image at the wafer level. MEEF may be generally defined as theratio of the critical dimension of a feature printed in a resist to thecritical dimension of a structure formed on a reticle.

As used herein, the term “wafer” generally refers to substrates formedof a semiconductor or non-semiconductor material. Examples of such asemiconductor or non-semiconductor material include, but are not limitedto, monocrystalline silicon, gallium arsenide, and indium phosphide.Such substrates may be commonly found and/or processed in semiconductorfabrication facilities.

A wafer may include one or more layers formed upon a substrate. Forexample, such layers may include, but are not limited to, a resist, adielectric material, and a conductive material. A “resist” may includeany material that may be patterned by an optical lithography technique,an e-beam lithography technique, or an X-ray lithography technique.Examples of a dielectric material may include, but are not limited to,silicon dioxide, silicon nitride, silicon oxynitride, and titaniumnitride. Additional examples of a dielectric material include “low-k”dielectric materials such as Black Diamond™ which is commerciallyavailable from Applied Materials, Inc., Santa Clara, Calif., and CORAL™commercially available from Novellus Systems, Inc., San Jose, Calif.,“ultra-low k” dielectric materials such as “xerogels,” and “high-k”dielectric materials such as tantalum pentoxide. In addition, examplesof a conductive material include, but are not limited to, aluminum,polysilicon, and copper.

One or more layers formed on a wafer may be patterned. For example, awafer may include a plurality of dies, each having repeatable patternfeatures. Formation and processing of such layers of material mayultimately result in completed semiconductor devices. As such, a wafermay include a substrate on which not all layers of a completesemiconductor device have been formed or a substrate on which all layersof a complete semiconductor device have been formed. The term“semiconductor device” is used interchangeably herein with the term“IC.” In addition, other devices such as microelectromechanical (MEMS)devices and the like may also be formed on a wafer.

Turning now to the drawings, it is noted that the figures are not drawnto scale. In particular, the scale of some of the elements of thefigures is greatly exaggerated to emphasize characteristics of theelements. It is also noted that the figures are not drawn to the samescale. Elements shown in more than one figure that may be similarlyconfigured have been indicated using the same reference numerals.

FIG. 1 illustrates one example of a configuration of dies printed onwafer 10 with a reticle for different values of a lithographic variable.In this example, reference or nominal dies N are printed on the wafer ata reference value for the lithographic variable that is being evaluated.The terms “reference die” and “nominal die” are used interchangeablyherein. The reference value may be the best known value for thelithographic variable (e.g., best dose, best focus, etc.).Alternatively, the reference value may be any predetermined baselinevalue.

The lithographic variable that is being evaluated may include anylithographic parameter that may alter the design, pattern that isprinted on the wafer by the reticle. Examples of such lithographicvariables include, but are not limited to, dose, focus, partialcoherence, and numerical aperture. It may be particularly desirable toevaluate the effect that different values of focus will have on thedesign pattern since this is typically the lithographic parameter thatwill change most often over time for a lithography process.

As also shown in FIG. 1, modulated dies M1, M2, and M3 are printed onthe wafer. The modulated dies are printed on the wafer at a value of thelithographic variable that is different than the reference value atwhich the nominal dies are printed. Although in the figure, the M1, M2and M3 dies are in different rows, this is not a restriction of theinvention. The different values at which the modulated dies are printedmay vary depending upon, for example, the degree to which thelithographic variable may be varied (e.g., the smallest increment changein the lithographic variable that can be made on the lithography tool),the typical process window for the lithographic variable, and/or thenumber of modulated dies that can be printed on the wafer (e.g., in thisexample, the number of rows of dies that can be printed on the wafer).In one particular example, it may be desirable to evaluate how thedesign pattern will be printed across the typical process window of thelithographic variable for a lithography process. Therefore, the range ofthe values of the lithographic variable to be evaluated may be dividedby the number of modulated dies that can be printed on the wafer todetermine appropriate increments in the different values of thelithographic variable. However, appropriate values for the lithographicvariable may be determined in any other manner.

Although three rows of dies are shown printed on the wafer in FIG. 1, itis to be understood that the number of rows of dies that are printed onthe wafer will vary depending on, for example, the dimensions of thedies and the dimensions of the wafer. In addition, although two sets ofdies (each set including an N-M-N sequence of dies) are shown in FIG. 1to make up each row of dies, it is to be understood that the number ofsets of dies in each row may also vary depending upon the dimensions ofthe dies and the dimensions of the wafer.

To inspect the design pattern, the dies in a row are imaged in a swath.The dies may be imaged using, for example, the wafer inspection systemdescribed herein or any other appropriate tool in the art such as waferinspection systems that are commercially available from KLA-Tencor, SanJose, Calif. The dies in a row may be imaged in the swath directionshown in FIG. 1 or in the opposite direction.

After imaging two adjacent dies printed on the wafer, the images of thetwo dies will be compared as shown by the arrows in FIG. 1. Inparticular, the first nominal die in the swath is imaged and saved.After imaging the adjacent M1 die, the images of the nominal die N andthe adjacent modulated die M1 are compared, and any differences betweenthe two dies are saved or otherwise noted, recorded, stored, etc. Thepresence of defects in the dies may then be determined using the resultsof the comparison. For example, to determine if the differences betweenthe two dies are defects, a threshold-type defect detection algorithmmay be applied to the difference data to determine if the differencesare indicative of defects.

The image of the M1 die may also be saved for comparison with the otheradjacent nominal die N after imaging of this nominal die in the swath.Images of these two dies may then be compared as described above, anddefects may be detected based on the results of the comparison asdescribed above.

Since the modulated M1 die is compared with two reference dies, theconfiguration shown in FIG. 1 allows double detection of defects in theM1 die. In other words, if a randomly caused defect appears in the firstreference die, then the differences between the images of the firstreference die and the M1 die may indicate the presence of a defect inthe M1 die even though the defect is actually present in the firstreference die. However, the probability that the randomly caused defectwill appear in the same position in the second reference die issubstantially low. Therefore, when the image of the M1 die is comparedto the second reference die, the defect that was found in the firstcomparison will most likely not be found in the second comparison. Assuch, defects that are found in only one of the two comparisons may belabeled as false defects and may be eliminated from any furtherevaluation.

Although the “double detection” of defects that is provided by comparingeach modulated die with two different nominal dies effectively reducesthe number of false defects that are detected, there are somedisadvantages to such methods. For example, a substantial amount ofspace on the wafer is used for printing nominal dies thereby reducingthe number of modulated dies that can be printed on the wafer, which inturn reduces the number of different values of the lithographic variablethat can be evaluated. Therefore, it would be advantageous to reduce thenumber of reference dies that are printed on the wafer without reducingthe accuracy of the defect detection method.

Several improvements to the above-described defect detection method aredescribed below. It is important to note that each improvement may beused alone or in combination with one or more of the other improvements.

One improvement can be realized by increasing the number of dies thatcan be imaged and processed simultaneously. For example, as describedabove, two dies are imaged (a nominal die and a modulated die), theimages of the two dies are compared to detect differences between theimages, and the differences are examined to identify those differencesthat indicate defects. Therefore, only two dies are processed at onetime. In an alternative, three dies may be imaged (two nominal dies andone modulated die), and these images may be processed simultaneously orin “real time” to detect defects in the modulated die.

It would be advantageous, however, if more image data could be processedsimultaneously. For example, according to one embodiment, the imagesthat are acquired may include images of an entire swath of dies printedon a wafer using the reticle. The images of the entire swath of dies canthen be examined by the inspection algorithm before flagging defects. Inother words, any useful or meaningful comparisons between any of thedies in the entire swath may be made prior to analyzing the differencesbetween the images for defect detection. In addition, the die layout inthe swath will be known a priori. In this manner, thecomputer-implemented method may select the appropriate die images forcomparison based on the position of the die images within the swath. Inanother embodiment, a user with knowledge of a layout of dies printed ona wafer can select which of the images are used for the comparison.

Although imaging an entire swath of dies at one time generates asubstantial amount of data that must then be handled as described hereinfor defect detection, image computers such as those described in U.S.patent application Ser. Nos. 10/967,388 to Bhaskar et al. filed Oct. 18,2004, now U.S. Pat. No. 7,149,642 issued Dec. 12, 2006, 10/967,397 toBhaskar et al. filed Oct. 18, 2004, now U.S. Pat. No. 7,076,390 issuedJul. 11, 2006, 10/967,542 to Bhaskar et al. filed Oct. 18, 2004, nowU.S. Pat. No. 7,555,409 issued Jun. 30, 2009, 10/967,419 to Bhaskar etal. filed Oct. 18, 2004, now U.S. Pat. No. 7,181,368 issued Feb. 20,2007, 10/967,375 to Blecher et al. filed Oct. 18, 2004, now U.S. Pat.No. 7,379,847 issued May 27, 2008, 10/967,838 to Bhaskar et al. filedOct. 18, 2004, now U.S. Pat. No. 7,024,339 issued Apr. 4, 2006,10/967,500 to Bhaskar et al. filed Oct. 18, 2004, now U.S. Pat. No.7,440,640 issued Oct. 21, 2008, 10/967,376 to Dubiner et al. filed Oct.18, 2004, 10/967,420 to Miller et al. filed Oct. 18, 2004, now U.S. Pat.No. 7,382,940 issued Jun. 3, 2008, 10/967,832 to Miller et al. filedOct. 18, 2004, and 10/967,418 to Bhaskar et al. filed Oct. 18, 2004, nowU.S. Pat. No. 7,602,958 issued Oct. 13, 2009, which are incorporated byreference as if fully set forth herein, may be used to handle such asubstantial amount of data.

Being able to simultaneously process image data from an entire swath ofdies on a wafer provides several advantages. For example, image data maybe acquired for the entire swath including two or more reference diesobtained at nominal values and one or more modulated dies. If two ormore reference dies are included in the swath, the methods describedherein may include generating a composite reference image from the twoor more reference images. For example, as shown in FIG. 2, a compositereference image may be generated from all four of the reference diesincluded in a swath. However, the composite reference image may begenerated from fewer than all of the reference dies in the swath. Inaddition, as shown in FIG. 2, a composite reference image may begenerated from the reference dies in one swath on the wafer, and othercomposite reference images may be generated for other swaths on thewafer. In this manner, a composite reference image may be generated inreal time after each swath is imaged. However, a composite referenceimage may alternatively be generated from two or more reference dies,and the same composite reference image may be used for defect detectionin modulated dies in the same or different swaths on the wafer.

The composite reference image may be generated in any manner known inthe art (e.g., averaging the image data of the two or more referencedies). In addition, it may be desirable to align the individualreference die images prior to generating the composite reference image.In one example, the reference die image frames in a swath may be alignedto a common coordinate reference, and any misalignment in the frames maybe corrected by sub-pixel interpolation of pixel values. The modulateddies may be aligned in a similar manner.

In any case, the composite reference image may be used for comparisonwith the modulated dies in the swath as shown by the arrows in FIG. 2.In other words, one of the at least two images used for comparison mayinclude the composite reference image. Using a composite reference imagefor defect detection exploits the presence of multiple nominal dies in aswath to improve the stability of the reference image against which eachof the modulated dies is compared. The use of a composite referenceimage may also improve the sensitivity of the detection by reducing theeffects of random noise that may be present in the individual referencedie images. In other words, the methods described herein areadvantageous in that the signal-to-noise ratio of the data used fordefect detection may be increased, which may in turn lead to the abilityto isolate the most likely relevant defects.

Using the composite reference image for comparison with the modulateddie images may also advantageously allow the number of reference diesincluded in the swath to be reduced. For example, as described above,double defect detection is advantageous in that it allows false defectscaused by defects in the reference die instead of the modulated die tobe eliminated from the detection results thereby increasing the accuracyof the defect detection methods. However, when the composite referenceimage is generated from two or more reference images, any differencesbetween the reference images may be detected, and these differences maybe analyzed to determine if defects are present in the reference images.Any defects that are determined to be present in one or more of thereference images may then be removed from the image data of theindividual reference dies. The “scrubbed” image data may then be used togenerate the composite reference image.

It is important to note that in the methods described herein, if anentire swath of die images can be generated and processedsimultaneously, the reference image that is used for comparison may bethe composite reference image as described herein or an individualreference image. Even if two or more non-composite reference images areused for comparison to modulated dies, the number of reference dies inthe swath can be reduced from the number currently being used in theN-M-N configuration since the individual reference die images can beused and reused for comparison to modulated die images regardless of theposition of the reference and modulated die images in the swath.

Using fewer nominal dies on a wafer advantageously allows more space onthe wafer to be used for modulated dies. Therefore, by using data moreefficiently and thoroughly, the methods described herein are able toinspect more examples of dies that are modulated and fewer examples ofreference dies. One other inspection technique that can be used toreduce the number of reference dies that are printed on a wafer is tocompare the modulated dies to a golden die image that is constructedfrom design information or prior scanning and then stored on some mediumsuch as a database. Efficient data use as described herein, however, ispotentially a more cost effective, accurate, and faster method thanusing golden die images from a database.

Unlike the configurations of the dies shown in FIGS. 1 and 2, when fewernominal dies can be used without reducing the accuracy of the detectionmethod, the number of modulated dies may be increased. One suchconfiguration is illustrated in FIG. 3. In this configuration, thenumber of modulated dies in a swath is equal to the number of referencedies in the swath. In addition, every other die position includes adifferent type of die in an N-M-N-M configuration. However, themodulated and nominal dies may be arranged in any other manner in theswath. For example, the first two dies in the swath may be referencedies, and all other dies in the swath may be modulated dies. In anycase, reducing the number of reference dies used by the method allowsmore modulated dies to be printed on a wafer thereby allowing the designpattern of the reticle to be examined for defects at more values of thelithographic variable being altered.

As shown in FIG. 3, the reference die images in a swath may be used togenerate a composite reference image. The composite reference image maybe generated as described above. In addition, the composite referenceimage may be used for comparison with the acquired images of themodulated dies as described above. In addition, as shown in FIG. 3,images of each of the reference dies in an entire swath may be used togenerate the composite reference image. Alternatively, images of fewerthan all of the reference dies in an entire swath may be used togenerate the composite reference image. Furthermore, as described above,a composite reference image may be generated for each swath on the waferthat is imaged. Alternatively, one composite reference image may begenerated and used for comparison to images of modulated dies in morethan one swath on the wafer.

As further shown in the configuration of FIG. 3, modulated dies in theentire swath may be printed using the same value of the lithographicvariable. In particular, each of the modulated dies in one swath are M1modulated, each of the modulated dies in another swath are M2 modulated,etc. In other words, if the lithographic variable that is beingevaluated is focus, each of the M1 modulated dies may be printed at thesame focus value, each of the M2 modulated dies may be printed at adifferent focus value that is the same for each M2 die, and so on. Thevalue of the lithographic variable used to print the dies in each swathis also preferably different than the value of the lithographic variableused to print the reference dies such that meaningful comparisons may bemade between the modulated dies and the reference dies.

Imaging an entire swath having the configuration shown in FIG. 3 andperforming defect detection as described above allows multiple similarlymodulated dies to be inspected at the same time. Performing defectdetection for more than one die modulated in a similar manner providesmore information about the design pattern and the defects detected inthe design pattern. For example, defects may be identified as randomlyoccurring defects if the defect shows up in fewer than all of thesimilarly modulated dies. Using the configuration shown in FIG. 3,therefore, an entire swath may be imaged, and these images may be usedto detect defects in dies having the same value of the modulatedlithographic variable.

A different configuration is illustrated in FIG. 4 in which themodulated dies in one swath have different values of the modulatedlithographic variable. In other words, the dies are laid out such thatthe modulation varies along a row rather than in a column as shown inFIGS. 1-3. In particular, the modulated dies in one swath may be M1modulated, M2 modulated, M3 modulated, and so on. In this manner, if thelithographic variable that is being evaluated is focus, the focus atwhich the M1 die is printed may be 0.1 μM, the focus at which the M2 dieis printed may be 0.2 μm, the focus at which the M3 die is printed maybe 0.3 μm, and so on. It is to be understood that these focus values aremerely intended to be examples of modulated focus values forillustrative purposes and are not to be interpreted as limiting orotherwise exemplary examples. The value of the lithographic variableused to print the modulated dies in each swath is also preferablydifferent than the value of the lithographic variable that is used toprint the reference dies such that meaningful comparisons may be madebetween the modulated dies and the reference dies.

Imaging an entire swath having the configuration shown in FIG. 4 andperforming defect detection as described above allows differentlymodulated dies to be inspected at the same time. Therefore, thisconfiguration may be advantageously used to examine defects in thedesign pattern across an entire range of values of the lithographicvariable in one swath. As such, one swath may be imaged and asubstantial amount of defect data may be generated from the imaged swathin a relatively short amount of time. In addition, since the swath mayinclude substantially more modulated dies than was previously available,the modulated dies in one swath may be printed at values of thelithographic variable spanning an entire typical process window for thereticle. In this manner, one swath may be imaged, and the swath imagemay be used to examine the process window of the reticle in asubstantially short amount of time, particularly when compared topreviously used process window qualification methods.

The configuration shown in FIG. 4 may also be used to examine the entiretrend of any pixel property as a function of modulation at each pixellocation (x, y). A “trend” may be generally defined as how acharacteristic of images at a particular pixel location such as pixelintensity varies as a function of different values of a lithographicvalue. As such, trends at particular pixel locations may be expressed bya plot such as those shown in FIG. 5. As shown in FIG. 5, a number oftrends that are relatively similar for a particular property at a pixellocation as a function of modulation may be defined as “typical trends.”Whether or not these “typical trends” are indicative of non-defectivepixel properties may be established in advance by another method (e.g.,defect review). The “typical trends” may be established experimentallythrough wafer or aerial image experiments or empirically throughsimulations (e.g., aerial image simulation).

Trends that appear to be atypical may be flagged as potential defects ofinterest or a potentially relevant defect location. In anotherembodiment, defects may be detected by comparing at least two imagesprinted or acquired at different values of a lithographic variable. Theimages may include images of modulated dies printed at different valuesof a lithographic variable and images of reference dies printed using anadditional different value of the lithographic variable as describedherein. In some embodiments, if a defect is determined to be present,the method may include assigning the defect to a group based on a trendin a plot of one or more characteristics of the images of the defect asa function of the different values of the lithographic variable.

The trend-based defect detection method described above is based on theassumption that line width variations and line-end pull backs that occuras a function of modulation will affect a larger number of pixels andfollow certain trends while the occasional “short” or other anomalousevents will occur in smaller numbers and follow a different trend.Therefore, it is clear that different defect detection methods may beused in the methods described herein to exploit information frommultiple modulated dies in a single swath. In addition, the trend-baseddefect detection method described above may be performed for differentlymodulated dies in a single swath or differently modulated dies indifferent swaths. In other words, the trend-based defect detectionmethod may be used regardless of the die configuration on the wafer.Furthermore, the trend-based defect detection method may advantageouslydetect defects of interest (DOIs) while ignoring the large number ofunimportant image differences (such as line width variations, line-endpull-backs, etc.) that will also occur as the lithographic variable ismodulated.

In another example of a trend-based defect detection method, apoint-to-point inspection on a relatively high resolution tool such as acritical dimension scanning electron microscope (CD SEM) or a Review SEMcould be used to perform measurements and/or defect detection from thenominal die outward in the modulated dies. In other words, apoint-to-point inspection based on the PWQ-type defect detection resultsmay be performed. This inspection of the PWQ-type defects may beperformed for the nominal dies and the modulated dies up to the point offailure in the design pattern. The corresponding points in the nominaland modulated dies that exhibit normal or expected variation ordegradation may be filtered out as non-defective or irrelevant. Anyremaining defects at these points may be classified (e.g., usingautomatic defect classification (ADC)) to look for bridging or otherdefect types that are relevant to process window errors. For example,for measurements such as CD measurements, the method may includedetermining if a “normal” variation in the CD measurements is present.This determination may be made using a recipe based on the predominantfeature direction (the predominant trend in the feature characteristicbeing measured). As such, relevant variations in the feature could bedistinguished from irrelevant variations in the feature. In a furtherexample, for defect detection, an ADC type inspection could be used tosearch for classic kinds of failure in the design pattern such asbridging features.

FIG. 4 a illustrates another die configuration that can be used asdescribed herein. In this configuration, exposure dose, E, can bemodulated in column 2. PWQ type defects can be detected by comparison ofthe dies in column 2 with a corresponding die in columns 0, 1, and/or 3.The exposure dose can also be modulated in column 5. Defects can bedetected by comparing the dies in column 5 with the correspondingreference dies in columns 3 and/or 4. In addition, exposure and dosemodulation may be examined on one wafer. For example, as shown in FIG. 4a, focus, F, may also be modulated in columns 7 and 10 on the wafer.Defects may be detected in these modulated dies by comparison of thedies with reference dies in the corresponding rows of columns 8, 9,and/or 11. In this manner, modulation of exposure dose and focus may beexamined separately on one wafer. The die configuration shown in FIG. 4a may be further configured as described herein.

Each of the die configurations described herein may be used by acomputer-implemented method to detect and/or sort defects in a designpattern of a reticle. In particular, the die configurations describedherein may be used in PWQ-type defect detection methods. For example, asdescribed above, images of the reticle for different values of alithographic variable may be acquired. In particular, acquiring theimages may include acquiring images of the design pattern printed on awafer using the reticle. These images may be acquired using, forexample, the system described herein. In addition, at least two of theimages may be compared. The method also include determining if a defectis present in the design pattern of the reticle using results of thecomparison.

As noted above, however, a relatively large number of unimportant orirrelevant image differences may be detected due to the very nature ofthe methods described herein. The large number of unimportantdifferences can result in the detection of a relatively large number ofirrelevant defects and false defects. The detection of irrelevant and/orfalse defects in such large numbers may have several disadvantages. Forexample, in order to identify the defects of interest, a user or asoftware program would have to sort through all of the irrelevant and/orfalse defects. Obviously, such sorting of the detected defects wouldreduce the throughput of the process of finding defects of interest.

As further noted above, the trend-based defect detection method may beused to differentiate between meaningful defects (or defects ofinterest) and irrelevant defects. It may also be advantageous to quicklyand accurately differentiate between defects of interest and irrelevantdefects after the defect detection has been performed. In other words,it may be desirable to perform defect classification to distinguishbetween defects of interest and irrelevant defects. One problem withcurrently used defect classification methods for use with the type ofdefect data that is generated as described herein is that the defectclassification methods tend to focus on characteristics of the defectsthemselves to identify the classification to which the defect belongs.In particular, due to the modulation of the lithographic variable, thesame defect may appear differently in differently modulated dies.Therefore, one defect may be assigned different classificationsdepending on the die in which it is detected.

According to one embodiment, a more accurate and useful defectclassification method for the PWQ based defect detection methodsdescribed herein may use one or more characteristics of a regionproximate the defect (i.e., the “background” information) to classifydefects. For example, the method may include isolating the immediateneighborhood of the background (which could be called a “micro-region”)and comparing the immediate neighborhood to others using standardcorrelation and template matching algorithms, which may be any suitablealgorithms known in the art of image processing. The micro-region may bedefined by a 16×16 pixel image centered on the defect or containing thedefect. Alternatively, the micro-region may be defined by a 32×32 pixelimage centered on the defect or containing the defect. In someembodiments, the region proximate the defect may be a “greaterneighborhood” region of about 64 pixels×64 pixels.

In another example, instead of using acquired image data to define thebackground proximate the defect, the defect location may be determinedin the GDS file of the design pattern (decorated or un-decorated withRET features). A portion of the design pattern data in the GDS fileproximate the defect may be selected. The background in the GDS file maybe compared to other defective locations as is done with the referencedie images. The additional locations can then be correlated to thedefect location from the original inspection. The additional locationsmay also be designated for review (e.g., by SEM).

In yet another example, the region proximate the defect may be generatedthrough aerial projection. In one such example, the aerial image datamay be taken from an aerial sensor such as that described in co-ownedU.S. patent application Ser. No. 10/679,857 filed Oct. 6, 2003 byStokowski et al. now U.S. Pat. No. 7,123,356 issued Oct. 17, 2006, whichis incorporated by reference as if fully set forth herein.Alternatively, the aerial image data could be generated by an aerialimage sensor of the type described in U.S. Pat. Nos. 6,803,554 to Ye etal. and 6,807,503 to Ye et al. and U.S. Patent Application PublicationNo. US 2003/0226951 by Ye et al., which is incorporated by reference asif fully set forth herein, which are incorporated by reference as iffully set forth herein.

By using the background features around and “behind” the defects, themethods described herein are able to find the relevant changes in thelithographic feature that can be lost in irrelevant defects using othermethods. As used herein, the term “background” refers to features of thereference image that are immediately “behind” the defect image (i.e.,the features of the reference image or the design pattern data that arelocated at the same pixel locations as the defect in the image of themodulated die) and the region around the defect image (i.e., thefeatures of the image of the modulated die proximate the defect). Inthis manner, by combining the results of “background binning” (e.g.,grouping defects on the basis of one or more characteristics of a regionproximate the defect) with PWQ type defect detection methods forordering the results of the repeating defect detection algorithm andprioritized die information, the methods described herein are able topresent the user with information that can be used to find critical orrelevant defects faster than existing defect detection methods.

According to one embodiment, therefore, a computer-implemented methodfor sorting defects in a design pattern of a reticle includes searchingfor defects of interest in inspection data using priority informationassociated with individual defects in combination with one or morecharacteristics of a region proximate the individual defects. The one ormore characteristics of the region (i.e., the background information)may be selected by a user. The inspection data is generated by comparingimages of the reticle generated for different values of a lithographicvariable. The images include at least one reference image and at leastone modulated image. In this manner, the method involves searching arelatively large amount of defect information for defects of interestusing the priority information generated by PWQ type inspection incombination with the background information. The priority informationcorresponds to a modulation level associated with the individualdefects.

The method also includes assigning one or more identifiers to thedefects of interest. In one embodiment, the one or more identifiers mayinclude an indicator identifying if the defects of interest are to besampled. In one such embodiment, assigning the identifier(s) may beperformed automatically based on the priority information and the one ormore characteristics of the region proximate the individual defects. Inanother embodiment, the identifiers may include one or more defectclassifications. The classifications may distinguish defect types usinguser defined names in some embodiments. Assigning identifier(s) to thedefects may be performed as further described herein.

In some embodiments, the method may include grouping the defects ofinterest based on the priority information, the one or morecharacteristics of the region proximate the individual defects, or acombination thereof. In another embodiment, the method may includegrouping the defects of interest based on the one or morecharacteristics of the region proximate the individual defects incombination with one or more characteristics of the defects of interest.Grouping the defects in these embodiments may be performed as furtherdescribed herein.

In this manner, the methods described herein may be used to find arelatively small number of defects of interest from a large amount ofcandidate defects. The inputs to the method may include defectpriorities, defect attributes, correlation to critical points from DRC,and defective and reference images. The candidate defects may befiltered based on the defect priorities and/or attributes to reduce thenumber of candidate defects that are searched. In some embodiments, thefeatures of the defects and the background may be obtained andcompressed for search. The outputs of the method may include defects ofinterest with class codes and review sample flags or any otheridentifiers known in the art. The defects that are not of interest maybe excluded from the inspection data such that the number of defectcandidates that are searched is reduced.

The embodiments of the method described above may include any othersteps described herein. For example, the method may include retrievingor finding similar defects based on search criteria and given defectexamples. In addition, the method may include performing a number offunctions to prepare for defect review such as providing status andfeedback to a user, generating charts for defect population in terms ofgroups, classes, and/or priorities, generating tags for defectpriorities and review samples, generating a defect list with defectinformation, and generating folders for classified or excluded defects.In addition, the method may include sampling defects for laterprocessing, which may be performed as described herein, and which mayreduce the number of defect samples that are reviewed or processed.Furthermore, as described further herein, the methods may be customizedby the user depending on, for example, the defects of interest bychanging criteria for filtering, grouping, retrieving, classifying,sampling, manually overriding the results of automated operations, andrepeating any step(s) at any time.

For example, in one embodiment, if a defect is determined to be presentin the design pattern of a reticle, the computer-implemented methodsdescribed herein may include assigning the defect to a group based onone or more characteristics of a region proximate the defect. Groupingcan be performed by either supervised or unsupervised classificationtechniques, which are known in the art of pattern, recognition. The oneor more characteristics of the region that are used for assigning thedefect to a group may include one or more characteristics of the designpattern in the region. In addition, the one or more characteristics ofthe region that are used for assigning the defect to a group may includeone or more characteristics of the region in one or more images used forthe comparison. In other words; the characteristic(s) of the region mayinclude the characteristic(s) of the region in the modulated die inwhich the defect was detected in addition to the characteristic(s) ofthe corresponding region in the one or more reference dies that werecompared with the modulated die. The characteristic(s) of the regionthat are used for sorting the defects into groups may also be selectedby a user. The user may select the characteristic(s) prior to groupingthe defects as described further herein.

In one embodiment, assigning the defects to a group may includecomparing an image of the region proximate the defect to images of theregions that are proximate to other defects detected in the designpattern. In another embodiment, a portion of a modulated die imageproximate the defect may be located in a GDS file image of the designpattern. The portion of the GDS file image corresponding to the portionof the modulated die image proximate the defect may be compared to othersimilar locations in the modulated die image.

In addition, the portion of the GDS file image or other design layoutmapped to the defect may be used to determine one or morecharacteristics of the region proximate the defect. Thesecharacteristics may be determined using any other images or data such asa high resolution image of the design pattern of the reticle. A highresolution image of the design pattern may be obtained using any highresolution reticle imaging system known in the art. In anotherembodiment, a simulated aerial image of the design pattern of thereticle may be used to determine one or more characteristics of theregion proximate the defect. The simulated aerial image may be generatedusing any suitable simulation program known in the art. In a differentembodiment, the one or more characteristics of the region proximate thedefect may be determined from an aerial image of the reticle obtainedusing an aerial imaging and measurement system (AIMS) as describedfurther herein.

Examples of methods that may be used for classification of the defectsare illustrated in U.S. patent application Ser. No. 10/954,968 to Huetet al. filed on Sep. 30, 2004, now U.S. Pat. No. 7,142,992 issued Nov.28, 2006, which is incorporated by reference as if fully set forthherein. Examples of additional methods that may be used for sorting andclassifying defects are illustrated in U.S. Patent Application Ser. No.60/618,475 to Teh et al. filed on Oct. 12, 2004, which is incorporatedby reference as if fully set forth herein.

After sorting the defects into groups, the method may include assigninga defect classification to one or more defects or the entire group. Thesame classification can be assigned to defects in different groups.Classification of the different groups of defects may include analyzingone or more characteristics of one or more defects in the group. Forexample, the method may include analyzing one or more characteristics ofone or more defects in the group to determine if the group of defects isan irrelevant defect group. The method may also include analyzing one ormore characteristics of one or more defects in the group to determine ifthe group indicates a failure in the design pattern. Classifying thedifferent groups of defects may also or alternatively include analyzingone or more characteristics of the background features around and behindthe defects.

The methods described herein may also include a number of otherfiltering and/or sorting functions. For example, the method may includecomparing the defects of interest to inspection data generated by designrule checking (DRC) performed on design pattern data of the reticle todetermine if the defects of interest correlate to DRC defects. In onesuch embodiment, the method may include removing from the inspectiondata the DRC defects that do not correlate with the defects of interest.

In such embodiments, the locations of the defect are correlated to knownvulnerable points based on the results of DRC. DRC can produce a list ofcritical points (sometimes referred to as “hot spots”). These points canbe used alone, directly as a guide for inspection and/or measurements ofreticle design pattern. However, the DRC often produces too many pointsfor inspection and/or measurement. Therefore, the critical pointsidentified by the DRC can be filtered as described herein using one ormore characteristics of a region proximate the critical points alone toreduce the population of the critical points. In addition, oralternatively, the critical points may be filtered using the “DefectsLike Me” function described herein to reduce the population. In thismanner, inspecting, measuring, and/or reviewing critical points that aresimilar may be reduced, and even eliminated.

In addition, the critical points identified by the DRC may be overlaidwith the inspection data generated as described herein. The inspectiondata may be data generated by imaging a wafer on which one or moremodulated dies and one or more reference dies are printed.Alternatively, the inspection data may include aerial images of thereticle design pattern generated by simulation or experimentation. Inthis manner, the defects of interest found as described herein may becompared to inspection data generated by design rule checking todetermine if the defects of interest correlate to design rule checkingdefects. The inspected defects that do not correlate with the DRCresults may then be removed from the inspection data. In each of theexamples provided above, ORC results may be used instead of DRC results.

The background binning methods described above have been shown to groupthe defects effectively such that the relevant defects can be foundfaster. In the case of PWQ methods, the background is sometimes the onlyrelevant feature group, and so during a PWQ experiment, the system canuse this feature set to group defects with similar backgrounds into thesame bins. These background features may be divided into a number ofdifferent subgroups (e.g., three subgroups), which may be presented tousers on a graphical user interface (GUI) such as those describedherein. The different subgroups may include, for example, statisticmeasures of image intensity, statistic measures of image intensityvariation, and measures of elementary image structures. Users can choosea combination of background subgroups to use in the PWQ binning.

FIG. 6 is a screenshot illustrating one example of a user interface thatcan be used to sort defects detected by the methods described herein. Inparticular, a user will be able to choose which subgroups of backgroundto use for binning of the defects, and FIG. 6 illustrates one possibleuser interface for selecting the subgroups. As shown in FIG. 6, the userinterface includes Defects Flow box 12, which includes a number ofoptions for the user. For example, Defects Flow box 12 includesFiltering by Priorities section 14. In this section, the user may selectthe defect priorities to use for filtering. The priorities may beselected individually by clicking on the boxes next to the prioritynumbers. Alternatively, the user may select all priorities or none ofthe priorities by clicking one of the buttons below the listing of theindividual priorities.

The PWQ defects are prioritized by the modulation level (e.g., M1, M2,M3, etc.) where they were first detected (as determined in the setup ofthe experiment) and within modulation by the number of occurrences ofthe defect found in all modulated dies through the repeater stacking ofall of the defects in the same modulation direction, positive ornegative from nominal. Such prioritization of the defects is furtherdescribed in U.S. Patent Application Publication No. US2004/0091142 toPeterson et al., which is incorporated by reference as if fully setforth herein. In the user interface, the user is able to filter by thispriority or select defects with certain priorities to work with inFiltering by Priorities section 14. Defects that do not fall within theselected priorities may be eliminated from the defect data.

In another embodiment, the defects may be filtered using one or morerules. The one or more rules may be based on, for example, one or morecharacteristics of the defects. In one embodiment, the user may createthe rules that are used to filter the defects. For example, as shown inFIG. 6 a, the user interface may include Filtering Rule box 13. TheFiltering Rule box allows the user to create a filtering rule in anumber of different ways. For example, the user may enter a ruledefinition in Rule Definition 13 a section of the Filtering Rule box. Inaddition, the user may select one or more elements 13 b in Build theRule section 13 c by checking the box beside those elements that are tobe used to filter defects. Although a number of different elements areillustrated in FIG. 6 a, it is to be understood that the elements thatare displayed in the Build the Rule section may vary depending on, forexample, the defect characteristics that are of interest.

Depending on the element that is selected, a number of differentoperators may be displayed in Operator section 13 d. The user may selectan operator to be used with the selected element. The user may selectthe operator by clicking on an operator or in any other manner known inthe art. In addition, the user may enter a value in Value section 13 ethat is to be used with the selected element and operator. The valuesthat are available for selection may vary depending on the element andthe operator that were previously selected. Once the user has selectedan element and operator, Histogram 13 f may be displayed in theFiltering Rule box. Histogram 13 f may illustrate the number of defectsfor different values of the element and operator. In this manner, theuser may be presented with information about the defects while buildingthe rule such that the user may tailor the rule to efficiently filterthe defects.

As further shown in FIG. 6, Defects Flow box 12 also includes GroupingRule section 16. The Grouping Rule section allows the user to selectwhich characteristics of the background and/or the defect are to be usedfor grouping or sorting of the defects. For example, as shown inGrouping Rule section 16, the user may select one or more backgroundfeatures or Context Features 18 for grouping. As shown in FIG. 6, theContext Features may include brightness, roughness, and pattern althoughthe Context Features that are available to the user may include anyother background features known in the art. In addition, although allthree Context Features are shown to be selected in FIG. 6, it is to beunderstood that the user may select fewer than all of the availableContext Features, any combination of the available Context Features, ornone of the available Context Features.

The user may also or alternatively select one or more Defect Features 20to be used for grouping of the defects. As shown in FIG. 6, the DefectFeatures may include size, shape, brightness, contrast, and background.However, the Defect Features that are available to the user may includefewer than all of these features. In addition, the Defect Features thatare available to the user may include any other appropriate feature(s)of defects that may be used for grouping. As further shown in FIG. 6,the user may select none of the Defect Features for grouping of thedefects. In particular, since the Defect Features may not necessarily beuseful for grouping of defects detected in the PWQ-type methodsdescribed herein, for the methods described herein, the user may notselect any of the Defect Features. However, the user may alternativelyselect one or more of the Defect Features to be used alone for groupingof the defects or to be used in combination with the Context Featuresfor grouping.

The Grouping Rule Section also includes Number of Groups option 22. Theuser may select or alter the number of groups into which the defects aresorted using the Number of Groups option. In this example, the user maytype a number of groups into the box, click the arrows next to the boxuntil the selected number appears, or move the arrow along the scaleuntil the selected number appears in the box. The number of groups thatare selected will affect how finely the defect and/or context featuresare divided among the groups. Therefore, a larger number of groups willresult in fewer and more similar defects assigned to each group. It isnoted that the number of groups may not be specified by the user.Instead, the algorithm can automatically determine an appropriate numberof groups.

Defects Flow box 12 also includes Sampling section 24 as shown in FIG.6. The Sampling section includes Number of Defects for Review option 26,in which the user can select a total number of defects for review.Review of the defects may be performed using any appropriate defectreview tool known in the art such as a SEM tool. The Sampling Sectionalso includes Defect Priorities to Sample option 28, in which the usercan select individual defect priorities that should be reviewed. Asshown in FIG. 6, the defect priorities may be selected for reviewindividually. However, the defect priorities may be selected in anymanner known in the art. In addition, the Sampling section includesDefect Classes to Sample option 30, in which the user can selectindividual defect classes that should be reviewed. The defect classesmay be selected for review individually as shown in FIG. 6 or in anyother manner known in the art. In addition, an automatic samplingalgorithm may be used to select defects for sampling using backgroundand priority information. In some embodiments, a list of sample sites tovisit and/or measure during review may be created based on the locationsof the defects in the modulated die(s). These locations may becorrelated to the location within the reticle such that the locationscan be found automatically by the review tool.

As further shown in FIG. 6, the Defects Flow box also includes a numberof buttons 32, which the user can select to apply the filtering,grouping, and sampling operations to the defect data. The user can applythese operations in any order. However, typically, a user may choose tofilter the defects before grouping them, and to group the defects beforesampling them for review. In this manner, the results of the backgroundbinning will be combined with the results of prioritized filtering sothat the user can view sampled defects by a combination of thebackground and priority. In addition, the filtering and groupingoperations can be performed iteratively rather than by fixed binningoperations.

The remaining boxes shown in the screenshot of FIG. 6 can be used todisplay the results of the filtering, grouping, and/or samplingoperations. However, these boxes may also be used to further work withthe defects. For example, the user interface shown in FIG. 6 includesDefect Runs and Classes box 34, in which a stacked color bar chart isillustrated showing the results of the filtering and groupingoperations. The stacked color bar chart can be used as a mechanism forillustrating and working with defect groups and priority together. Eachbar represents a group of defects. Color indicates defect priorities.Although such a chart may advantageously illustrate a substantial amountof information about the defects in a relatively easy-to-comprehendmanner, it is to be understood that any method or graphical structuremay be used to illustrate the results of the filtering and/or groupingoperations.

For example, the bar chart illustrated in FIG. 6 was displayed since theGrouped Defects option 34 a was selected. However, if the FilteredPopulation and Defect Grid options are selected, different graphics willbe displayed. For example, FIG. 6 b illustrates another manner in whichthe results of filtering and grouping can be displayed when Defect Gridoption 34 b is selected. As shown in FIG. 6 b, the Defect Runs andClasses box 34 may include grid 34 c that illustrates the number ofdefects that were found as a function of priority and group. Although acertain number of priorities and groups are illustrated in FIG. 6 b,obviously the number of priorities and groups will vary depending on theparameters used for filtering and grouping.

FIG. 6 c illustrates a different manner in which the results offiltering and grouping can be displayed when Filtered Population option34 d is selected. As shown in FIG. 6 c, the number of defects that werefound as a function of priority are illustrated in bar chart 34 e.However, it is to be understood that the number of defects as a functionof priority may be illustrated in any other manner known in the art. Inaddition, although three different priorities are illustrated in FIG. 6c, it is to be understood that the number of priorities will varydepending on the parameters used for filtering.

The user interface shown in FIG. 6 may also include Available Defectsbox 36. The available defects box may illustrate verification defects.For example, the available defects box may illustrate the results fromfiltering, grouping, and retrieving. All defects which are notclassified or not in classified defects folders can be displayed in anarea. As shown in FIG. 6, the Available Defects box may illustrateimages of the defects. Alternatively, the Available Defects box mayprovide information about the results using any suitable method known inthe art. In addition, the user may perform one or more functions on theavailable defects using the Available Defects box.

The user interface may illustrate results of filtering and groupinggraphically as described above and with images of defects selected forsampling. For example, as shown in Sample Defects box 38, the userinterface may illustrate a folder into which defects were classified. Inaddition, if defects were assigned to the folders, representative defectimages may be illustrated on the front of the folders. The user mayperform a number of functions on the defect images shown in the SampleDefects box. For example, the user may select one of the folders intowhich defects were assigned. Selecting one of the folders may result inillustration of the defect images in the selected folder in box 40 belowthe illustration of the different folders. As shown in box 40, thedefect images may also be illustrated with a number. The number mayindicate the priority assigned to each defect image.

The user can move defects from one folder into another to change defectclassification. The user can also perform un-classification by movingdefects into the Available Defect Gallery. The user can add folders,delete the folders and rename the folders. Deleting a folder willun-classify all defects in that folder. The first folder, calledDefects-to-ignore, on the left is a folder for all defects that areexcluded from filtering, grouping and sampling. One or more such folderscan exist. Moving defects into classified folders can be achieved, butnot limited by, selecting followed by dragging and dropping the defectsinto the folders, or by selecting followed by clicking a button, likethe Defects-to-ignore button. Although one manner of illustrating thesample images to a user is illustrated in FIG. 6, it is to be understoodthat any other manner of illustrating sample images may be used in theuser interface and methods described herein.

FIG. 6 d illustrates another example of Sample Defects box 38. As shownin Sample Defect box 38 of FIG. 6 d, the folder for Class 2 of thedefects has been selected resulting in illustration of the defect imagesin the Class 2 folder in box 40 below the illustration of the differentfolders. Sampling can be performed either automatically or manually. Byclicking the Apply Sampling button of buttons 32 shown in FIG. 6, thedefects in classified folders are sampled according to the criteria setfor sampling. The user can also select one or more defects in classifiedfolders and mark them as sampled defects by clicking Mark Selected orMark All, as shown in FIG. 6 d. All sampled defects may be tagged with amarker. As further shown in FIG. 6 d, the user can turn off the samplestatus for one or more defects by using the button, Unmark Selected orUnmark All.

The sample images may also be illustrated to the user in other manners.For example, the user interface may be configured to display any of thedefects or just the sample images intermittently with reference imagescorresponding to the defect images. In this manner, the images mayappear to flash in the user interface repeatedly one after the other.Such “flashing” of the images may allow the user to gain additionalunderstanding of the differences between the images. In a similarmanner, sample images of differently modulated dies may be flashed inthe user interface, which may aid in user understanding of trends of thedefects.

It is also noted that although the user interface is shown to includefour different boxes in FIG. 6, it is to be understood that the userinterface may include fewer than four information boxes or more thanfour information boxes. In general, the amount and organization of theinformation shown in the user interface may be designed to present themaximum amount of information to a user in the most manageable andeasy-to-comprehend manner possible.

The user interfaces described herein provide a number of advantages incomparison to other currently used user interfaces for processinginspection results. Particularly, as described further above, the userinterface provides pre-filtering capability, which may be performedbased on priority and/or rules. The parameters of pre-filtering may beselected by a user as described further above. In addition, thebackground characteristic(s) that are used for grouping may also beselected as described herein. The background characteristic(s) may alsobe used with other defect attributes for grouping as described above.Furthermore, the user interface can be used to perform iterativegrouping rather than fixed binning. An automatic sampling algorithm mayalso be used with the background grouping and priority filteringresults. The functionality of the user interface may also be expanded,for example, to create a list of sample sites to visit and/or measurebased on the locations of modulated die and then to make a “fake” resultto be used.

As interesting defects are found, the user can also use a different userinterface to see other examples of defects that are similar to theinteresting defects using a defect retrieving feature called “DefectsLike Me.” The user can also use this feature to remove groups ofirrelevant defects in order to quickly traverse through the large numberof defects. One example of a user interface that can be used toillustrate defects that are similar is shown in FIG. 7. As shown in FIG.7, the user interface includes Sample Defect box 42, which illustratesthe defect that is selected by the user. The user may select this defectfrom the defects illustrated in another user interface such as thatshown in FIG. 6. As also shown in FIG. 7, the user may perform one ormore functions on the image of the selected defect using icons 44.

The user interface also includes Searching Criteria box 46, in which theuser can select one or more parameters for searching for defects thatare similar to the selected defect. In particular, Searching Criteriabox 46 includes Manual Features Selection section 48. In the ManualFeatures Selection section, the user can select one or more features ofthe defects to use for searching for similar defects. As shown in FIG.7, the features that can be selected include size, brightness, shape,contrast, polarity, and context. However, it is to be understood thatthe features which are available for selection may include anyappropriate features known in the art.

As shown in Manual Features Selection section 48, the user may alsoselect all of the features or none of the features by clicking on theappropriate button. Alternatively, the user may manually selectindividual features by clicking on the boxes next to the feature name.Although only the context feature is shown to be selected in FIG. 7, itis to be understood that any of the other features may alternatively beselected or a combination of features may be selected. As describedabove, the context or background of the defects may be advantageouslyused to group defects since the features of the defects themselves mayactually vary greatly from one modulated die to another. Therefore, theselected context feature may frequently be used to search for similardefects.

As further shown in FIG. 7, a weight may be assigned to each of thefeatures to be used for searching for similar defects. The weightassigned to each feature may be a default weight assigned automaticallyor upon selection of the appropriate button. Although each of thedefault weights are shown to be the same, it is to be understood thatthe default weights for individual features may vary. The user mayassign different weights to individual features in a number of differentmanners. For example, the user may type a number for the weight into thebox, click the arrows next to the box until the selected weight appears,or move the arrow along the scale until the selected weight appears inthe box.

As shown in FIG. 7, Searching Criteria box 46 also includes Sensitivitysection 50, in which the user may select the sensitivity with whichdefects are to be searched. The sensitivity may be selected in differentways. For example, as shown in FIG. 7, the sensitivity may be defined bythe number of defects to which the search results are limited. In otherwords, the number of defects shown in Sensitivity section 50 mayindicate to the computer-implemented method that the searching resultsare to be limited to 50 defects (or some other number of defects) thatare most like the selected defect. Alternatively, the user may definethe sensitivity of the search by assigning a threshold to the featuresthat are selected for the search. Although only one threshold is shownin FIG. 7, it is to be understood that the number of threshold optionsthat are shown in FIG. 7 may vary depending on the number of featuresthat are selected for searching.

Searching Criteria box 46 also includes Start Searching button 52 whichthe user can click once the appropriate choices have been made in theSearching Criteria box. During or after searching, images of the defectsthat are determined to be like the selected defect based on thesearching criteria may be illustrated in Found Defects section 54 of theuser interface. As shown in FIG. 7, the user may perform a number ofdifferent functions on the defect images using icons 56. In addition,the user may select to accept the found defects using Accept button 58.Alternatively, the user may decide to quit the “Defects Like Me”function using Quit button 60.

It is noted that although the user interface is shown to include threedifferent boxes in FIG. 7, it is to be understood that the userinterface may include fewer than three information boxes or more thanthree information boxes. In general, the amount and organization of theinformation shown in the user interface may be designed to present themaximum amount of information to a user in the most manageable andeasy-to-comprehend manner possible.

In additional embodiments, the methods described herein may includealtering the design pattern on the reticle based on the results of thedefect detection and/or sorting methods described herein. In particular,the results of the methods described herein may be used to determine ifthe reticle passes qualification standards for the reticle. If thereticle does not pass qualification, then the reticle design pattern maybe altered. Preferably, the reticle design pattern is altered such thatfewer defects in the design pattern will be produced in the designpattern printed on the wafer. A new reticle may then be fabricated withthe altered design pattern. Alternatively, in some instances, thereticle may be physically altered to alter the design pattern on thereticle. Physically altering the reticle may be performed using anyrepair process known in the art such as focused ion beam repairprocesses.

In another embodiment, the methods described herein may includegenerating a different design pattern for the reticle based on theresults of the defect detection and/or sorting methods described herein.In particular, a new design pattern may be generated if the designpattern that was inspected was found to have a substantially largeamount of defects, a relatively large number of defects that cannot befixed, and/or defects that cannot be fixed and will cause fatal flaws inthe design pattern that will be printed on the wafer. In yet anotherembodiment, the results of the methods described herein may be fedforward to the design process of other reticles. In particular, theresults of the methods described herein may be used to design RETfeatures in other reticles.

Some embodiments of the method may include determining a process windowof the reticle. For example, it may be determined if some smaller rangeof the value of the lithographic variable that was examined can be usedto adequately reproduce the design pattern on wafers. In this manner,the reticle may be qualified for use with a smaller than normal processwindow. The degree to which the process window can be narrowed in anacceptable manner will vary depending on, for example, the drift in thelithographic variable that can be expected for lithography systems thatwill use the reticle. In this manner, a defective reticle design patternmay be used without fixing the defects in the reticle design pattern.

Program instructions implementing methods such as those described hereinmay be transmitted over or stored on the carrier medium. The carriermedium may be a transmission medium such as a wire, cable, or wirelesstransmission link, or a signal traveling along such a wire, cable, orlink. The carrier medium may also be a storage medium such as aread-only memory, a random access memory, a magnetic or optical disk, ora magnetic tape.

The program instructions may be implemented in any of various ways,including procedure-based techniques, component-based techniques, and/orobject-oriented techniques, among others. For example, the programinstructions may be implemented using Matlab, Visual Basic, ActiveXcontrols, C, C++ objects, C#, JavaBeans, Microsoft Foundation Classes(“MFC”), or other technologies or methodologies, as desired.

The processor may take various forms, including a personal computersystem, mainframe computer system, workstation, network appliance,Internet appliance, personal digital assistant (“PDA”), televisionsystem or other device. In general, the term “computer system” may bebroadly defined to encompass any device having one or more processors,which executes instructions from a memory medium. In addition, theprocessor may include a processor as described in the patentapplications incorporated by reference above, which are particularlysuitable for handling a relatively large amount of image datasubstantially simultaneously.

FIG. 8 illustrates one embodiment of a system configured to perform oneor more of the computer-implemented methods described herein fordetecting and/or sorting defects. The system shown in FIG. 8 isconfigured to inspect a wafer. Although the system is shown in FIG. 8 tobe an optical based imaging system, it is to be understood that thesystem shown in FIG. 8 may be configured to image the wafer in adifferent way. For example, the system may be configured to inspect awafer by imaging the wafer with electron beams (i.e., an electron beambased imaging system or SEM).

The system includes processor 62. The processor may include any suitableprocessor known in the art. For example, the processor may be an imagecomputer or a parallel processor. In addition, the processor may beconfigured as described above. The system also includes carrier medium64. The carrier medium may be configured as described above. Forexample, carrier medium 64 includes program instructions 66, which areexecutable on processor 62. The program instructions may be executablefor performing any of the embodiments of the methods described above.The program instructions may be further configured as described above.

In some embodiments, the system may also include inspection and/orreview tool 68. Tool 68 may be configured to image wafer 70 and togenerate image data for the wafer that contains information about thedesign pattern printed on the wafer by a reticle. Tool 68 may be coupledto processor 62. For example, one or more components of tool 68 may becoupled to processor 62 by a transmission medium (not shown). Thetransmission medium may include “wired” and “wireless” portions. Inanother example, detector 72 of tool 68 may be configured to generateoutput 74. The output may be transmitted across a transmission mediumfrom detector 72 to processor 62. In some embodiments, the output mayalso be transmitted through one or more electronic components coupledbetween the detector and the processor. Therefore, output 74 istransmitted from the tool to the processor, and program instructions 66may be executable on the processor to detect and/or sort defects on thewafer as described herein using the image data included in output 74.Program instructions 66 may be further executable on the processor toperform other functions described herein (e.g., “Defects Like Me”searching, sorting defects by priority, selecting defects for sampling,etc.).

Inspection and/or review tool 68 may be configured to generate images ofthe wafer using any technique known in the art. In addition, the toolincludes stage 76 upon which wafer 70 may be disposed during imaging ormeasurements. The stage may include any suitable mechanical or roboticassembly known in the art. The tool also includes light source 78. Lightsource 78 may include any appropriate light source known in the art. Inaddition, the tool may include beam splitter 80, which is configured todirect light from light source 78 onto wafer 70 at angles that areapproximately normal to an upper surface of wafer 70. The beam splittermay include any suitable beam splitter known in the art. The toolfurther includes detector 72, which is configured to detect lighttransmitted by beam splitter 80. The detector is also configured togenerate output 74. The detector may include any suitable detector knownin the art.

Although one general configuration of the inspection and/or review toolis shown in FIG. 8, it is to be understood that the tool may have anysuitable configuration known in the art. For example, the tool may beconfigured to perform a single channel imaging technique as shown inFIG. 8. Alternatively, the tool may be configured to perform a multiplechannel imaging technique. In addition, the optical tool may be replacedwith an e-beam inspection tool such as a CD SEM and the eS25 and eS30systems, which are commercially available from KLA-Tencor. Such a toolmay be coupled to the processor as described above.

In another embodiment, the computer-implemented methods described abovemay be performed using aerial images. For example, the methods describedherein may be implemented using an aerial image measurement system(AIMS) technique, which may be better understood by reference to FIG. 9.In FIG. 9, a system is shown having three detectors, i.e., detectors101, 102 and 103. Each of these detectors may preferably be set at adifferent focal position. For example, detector 101 could be at zerodefocus, detector 102 could be at +0.2 defocus, and detector 103 couldbe at −0.2 defocus. Of course, these levels of defocus are onlyexamples. Any suitable range or levels of defocus could be used, andsuch levels could be optimized empirically. It is not necessary to use adetector having zero defocus, for example, and all of the detectorscould be set at varying levels of positive defocus, or at mixed levelsof positive and negative defocus.

Sample 104 is preferably a mask or reticle. As sample 104 is exposed toillumination source 105, an aerial image is detected at the threedetectors. Because of their different focal positions, the aerial imagesat each detector will have different levels of defocus. Images havingvarying levels of defocus may be compared and analyzed using any of thetechniques previously set forth herein. In a preferred embodiment,signals taken from a first detector, such as detector 101, are comparedto signals taken from a second detector, such as detector 102,continuously as sample 104 is inspected. This is only one example, ofcourse, and images from any pairs of detectors could be compared.Alternatively, comparisons could be made between detectors andmathematical combinations of other detectors (such as a pixel by pixelaverage between a pair of detectors, or a difference between anotherpair of detectors). Preferably, the levels of defocus and/or the typesof comparisons between the signals from the various detectors (orcombinations thereof) are selected to provide the user with informationregarding RET defects and the appearance of such defects across aprocess window.

In the embodiment shown in FIG. 9, it is possible to simultaneouslyperform a conventional inspection and a process window qualification.The purpose and methodology of the process window qualification (to findRET defects and the like) has already been described herein. The purposeof the conventional inspection is to find other types of defects, suchas defects resulting from reticle manufacturing errors and/or fromcontaminants on the reticle. A method of such a conventional inspectionis described in U.S. Pat. No. 6,268,093 to Kenan et al., which isincorporated by reference as if fully set forth herein. Other suitablemethods of performing such inspections are described in more detail in acommonly assigned application by Stokowski et al. having U.S. Ser. No.10/679,617, filed Oct. 6, 2003, now U.S. Pat. No. 7,379,175 issued May27, 2008, which is incorporated by reference herein in its entirety andfor all purposes. Such suitable methods include, without limitation, adie-to-database inspection in which the reticle is inspected bycomparison against a rendered database from which the reticle wascreated.

In a preferred embodiment, the conventional inspection is done bycomparing signals from the same detector taken at nominally identicalportions of different dies. This inspection process works well formulti-die reticles. The process window qualification is performedsubstantially simultaneously, and may be achieved as already describedherein by comparing images at varying levels of defocus for each die. Sothe conventional inspection might be achieved by comparing images from afirst die on sample 104 to images of a second die on sample 104, whereineach image is detected using detector 101. At substantially the sametime as the images of each such die are collected for purposes of theconventional inspection, for each such die an image from detector 101and/or detector 102 or detector 103, is also compared to an image ofthat same die taken at a different focal position (for example fromanother of detectors 101, 102 and/or 103, or any mathematicalcombination thereon). Thus, the conventional inspection and processwindow qualification may be performed substantially simultaneously.

If desired, the processing of the data from the conventional inspectionand from the process window qualification could be performed on the samecomputer by using parallel processing. A suitable architecture andmethodology are described in more detail in a commonly assignedapplication by Goldberg et al. having U.S. Ser. No. 09/449,022, filedNov. 24, 1999, now U.S. Pat. No. 7,106,895 issued Sep. 12, 2006, whichis incorporated by reference herein in its entirety and for allpurposes.

In yet another embodiment of the invention, and in accordance with theabove description of the example shown in FIG. 9, a single die reticlecould be provided as sample 104, and only a process window qualificationmay be performed using the apparatus shown in FIG. 9. Such a techniquemay be desirable for all types of reticles, and may be particularlydesirable for single die reticles. This is because the apparatus shownin FIG. 9 is in many ways inferior to other types of inspection systems,such as the 3XX and 5XX series commercially available from KLA-TencorCorporation. Thus, it may be desirable to find conventional defectsusing the KLA-Tencor tools, and then inspect the same reticle again inan aerial image mode to locate RET defects by varying the processwindow. As mentioned above, this may be particularly desirable wheresample 104 is a single die reticle. This avoids the need to render thedesign database in a mode suitable for comparison against the aerialimage. Instead, the aerial image is used only for purposes of findingRET defects, and the conventional inspection is done using a moreaccurate tool which can directly compare the actual image of the reticleto the rendered database (including the OPC features present therein).

Of course, if a suitably rendered database is available for comparisonagainst the AIMS image (rendered using the techniques described, forexample, in the application by Stokowski et al., as mentioned above), adie-to-database inspection could be done using an AIMS tool such as thatshown in FIG. 9. In such a case, it is possible to also do theinspection for RET defects by using a comparison against the rendereddatabase. For example, the conventional inspection could be performed bycomparing images from a detector at zero defocus to images rendered fromthe database, also at zero defocus. The RET defects could then be foundby comparing the images from one or more detectors, at varying levels ofdefocus, against the rendered database at zero defocus. Or the databasecould also be, through simulation, rendered in a manner that isconsistent with a given level of defocus. In either situation, themethods described herein could be applied to find RET defects.

The present invention is not limited to just finding RET defects byvarying the level of defocus. As noted above, varying sigma and/or thenumerical aperture (NA) of the system are also relevant to the processwindow. Varying these parameters can, therefore, be used to find RETdefects. One method of achieving this is to take an image obtained usingan inspection under a first set of conditions (i.e., a first set ofsigma, NA and defocus), then take an image of the same reticle under asecond set of conditions (i.e, varying one or more of the NA, sigma anddefocus), and compare the resulting images. Such a method can beimplemented, using an apparatus such as that shown in FIG. 9, simply bystoring data taken from a first inspection of a reticle under a firstset of conditions, varying parameters such as sigma, NA and/or defocuson the apparatus, and then re-inspecting the same reticle with the newparameter settings in place. The images are aligned prior to comparison.The stored data could be taken from inspection of an entire reticle (andstored on an optical disk or other media having suitable storage space),or could be taken across just a portion of the reticle (such as one ormore swaths). If only a portion of the reticle inspection data isstored, storage might be appropriately handled in a memory buffer or thelike. In some embodiments, the stored data may represent a “referencereticle field,” or an aerial image of the reticle that would be producedat the best known process conditions, which may be stored such that itcan be later used for transient repeating defect detection and/ornon-transient defect detection.

In another embodiment, stored data could be taken from inspection of anentire die or just a portion of the die. In one such embodiment, the dieor the portion of the die may correspond to a design pattern that isformed on the wafer using a reference value of a lithographic variable,which in some embodiments may be the best known conditions. In thismanner, the stored data may represent a “reference die.” In alternativeembodiments, the stored data may be a simulated image. For example, thesimulated image may be an image that would be printed on the wafer atthe reference member value. In one embodiment, the simulated image maybe generated from reticle design data. The reticle design data may bealtered based on the reference value to generate a simulated aerialimage of the reticle. In a different embodiment, the simulated image maybe generated from an aerial image of the reticle that is acquired byreticle inspection. The simulated aerial image or the acquired aerialimage may be altered using a resist model to generate an image of thereticle that would be printed on the wafer at the reference value.

The stored data may be compared to other die or portions of die on thewafer to determine a presence of defects on the wafer. In someembodiments, the die that are compared to the stored data may be printedat different conditions (i.e., not the reference value). As such, thestored data may be used to determine a presence of transient repeatingdefects in the die or the portions of the die on the wafer.Alternatively, the die that are compared to the stored data may beprinted at the same conditions as the stored data (i.e., the referencevalue). Therefore, the stored data may be used to determine a presenceof non-transient defects in the die or the portions of the die on thewafer.

As shown in FIG. 9, the system may include a number of other componentsincluding, but not limited to, homogenizer 106, aperture 107, condenserlens 108, stage 109, objective lens 110, aperture 111, lens 112,beamsplitter 113, and processor or computer 114. The components may beconfigured as described in more detail in a commonly assignedapplication by Stokowski et al. having U.S. Ser. No. 10/679,617, filedOct. 6, 2003, now U.S. Pat. No. 7,379,175 issued May 27, 2008. Thesecomponents may be altered to provide varying parameters such as sigma,NA, the type of illumination, and the shape of the beam. For example,aperture 107 may be altered to change sigma, the NA, the type ofillumination, and the shape of the beam.

In a preferred embodiment, rather than directly comparing raw data fromeach detector (and/or from a rendered database), it may desirable topreprocess the data prior to comparison, as described in U.S. PatentApplication Publication No. US2004/0091142 to Peterson et al., which isincorporated by reference as if fully set forth herein.

In another preferred embodiment, the data taken from inspection by anymethod described herein (e.g., inspection using aerial images,inspection of images printed on a wafer, inspection of simulated imagesin accordance with DRC techniques, etc.) may be used to flag regions ofa reticle or wafer for review. The defects may be selected for review asdescribed above. The coordinates for such review could be stored by theinspection apparatus and passed to a review tool (or performed on areview tool integrated into the inspection apparatus). In one preferredembodiment, the review tool is an aerial image review tool of the typecommercially available from Carl Zeiss, Inc., Germany. Potential RETdefect locations on a reticle are identified, and the coordinates arepassed to the Zeiss tool. Each such potential defect (or a samplestatistically selected from a group of such defects) is then reviewed atvarying levels of defocus (or other optical conditions, such as sigma orNA) to further study the possible defect and its potential significance.

It is to be noted that the above methods that use aerial images may alsobe performed in a similar manner using simulated images (e.g., imagesacquired using DRC techniques or ORC techniques).

Further modifications and alternative embodiments of various aspects ofthe invention may be apparent to those skilled in the art in view ofthis description. For example, computer-implemented methods fordetecting and/or sorting defects in a design pattern of a reticle areprovided. Accordingly, this description is to be construed asillustrative only and is for the purpose of teaching those skilled inthe art the general manner of carrying out the invention. It is to beunderstood that the forms of the invention shown and described hereinare to be taken as the presently preferred embodiments. Elements andmaterials may be substituted for those illustrated and described herein,parts and processes may be reversed, and certain features of theinvention may be utilized independently, all as would be apparent to oneskilled in the art after having the benefit of this description of theinvention. Changes may be made in the elements described herein withoutdeparting from the spirit and scope of the invention as described in thefollowing claims.

1. A computer-implemented method for detecting defects in a designpattern of a reticle, comprising: acquiring images of the reticle fordifferent values of a lithographic variable, wherein the images comprisetwo or more reference images obtained at nominal values and one or moremodulated images; generating a composite reference image from the two ormore reference images; comparing at least two of the images, wherein theat least two of the images comprise the composite reference image; anddetermining if a defect is present in the design pattern using resultsof said comparing.
 2. The method of claim 1, wherein if the defect isdetermined to be present, the method further comprises assigning thedefect to a group based on one or more characteristics of a regionproximate the defect.
 3. The method of claim 2, wherein the one or morecharacteristics of the region comprise one or more characteristics ofthe design pattern in the region of at least one of the two or morereference images.
 4. The method of claim 2, wherein the one or morecharacteristics of the region comprise one or more characteristics ofthe region in the at least two of the images used for said comparing. 5.The method of claim 2, wherein the one or more characteristics of theregion comprise one or more characteristics of the region extracted froma GDS or aerial image.
 6. The method of claim 2, wherein the one or morecharacteristics of the region comprise one or more characteristics ofthe region determined from a high resolution image.
 7. The method ofclaim 2, further comprising analyzing the one or more characteristics ofthe region of one or more defects in the group to determine if the groupis an irrelevant defect group.
 8. The method of claim 2, furthercomprising analyzing one or more characteristics of one or more defectsin the group to determine if the group indicates a failure in the designpattern.
 9. The method of claim 1, wherein the images further compriseimages of an entire swath of dies printed on a wafer using the reticle,and wherein the at least two of the images used for said comparingfurther comprise the images of all of the dies in the entire swath. 10.The method of claim 9, wherein modulated dies in the entire swath areprinted using the same value of the lithographic variable, which isdifferent than the value of the lithographic variable at which referencedies are printed in the entire swath.
 11. The method of claim 9, whereinmodulated dies in the entire swath are printed using the differentvalues of the lithographic variable, and wherein reference dies in theentire swath are printed using an additional different value of thelithographic value.
 12. The method of claim 1, wherein a user withknowledge of a layout of dies printed on a wafer selects which of the atleast two of the images used for said comparing.
 13. The method of claim1, wherein said acquiring comprises acquiring images of the designpattern printed on a wafer using the reticle.
 14. A computer-implementedmethod for detecting defects in a design pattern of a reticle,comprising: acquiring images of an entire swath of dies printed on awafer using the reticle, wherein at least two of the dies are printed atdifferent values of a lithographic variable; subsequent to saidacquiring, comparing at least two of the images; and determining if adefect is present in the design pattern using results of said comparing.15. The method of claim 14, wherein the dies comprise modulated dies andat least one reference die, and wherein a number of the modulated diesin the entire swath is greater than or equal to a number of the at leastone reference die in the entire swath.
 16. The method of claim 14,wherein the dies comprise two or more reference dies, wherein the methodfurther comprises generating a composite reference image from the imagesof the two or more reference dies, and wherein one of the at least twoof the images used for said comparing comprises the composite referenceimage.
 17. The method of claim 14, wherein the dies comprise modulateddies and at least one reference die, and wherein the modulated dies areprinted at values of the lithographic variable that are the same foreach of the modulated dies but different than the value of thelithographic variable at which the at least one reference die isprinted.
 18. The method of claim 14, wherein the dies comprise modulateddies and at least one reference die, and wherein the modulated dies areprinted at values of the lithographic variable that are different foreach of the modulated dies and different than the value of thelithographic variable at which the at least one reference die isprinted.
 19. A computer-implemented method for detecting and sortingdefects in a design pattern of a reticle, comprising: acquiring imagesof the reticle for different values of a lithographic variable;comparing at least two of the images; determining if pixel differencesin the at least two of the images follow a typical or atypical trendover the different values of the lithographic variable.
 20. The methodof claim 19, wherein the atypical trend indicates a potentially relevantdefect location.
 21. The method of claim 19, wherein the images compriseimages of modulated dies printed at the different values of thelithographic variable and images of reference dies printed using anadditional different value of the lithographic variable.
 22. The methodof claim 19, wherein said acquiring comprises acquiring images of thedesign pattern printed on a wafer using the reticle.